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United States Patent |
6,049,184
|
Uggla
,   et al.
|
April 11, 2000
|
Method and arrangement for controlling a current
Abstract
To control the current through a winding interconnected in an H-bridge to
control e.g. a stepping motor, at the beginning of each chopper period,
the H-bridge is kept in its slow demagnetization state during
predetermined interval (t1-t2). Then, just before, at or just after the
end of that interval, the actual current value (IA) is compared with a
desired value (ID). If the actual value (IA) is smaller, the H-bridge is
kept in its magnetization state, at the most until the end of the chopper
period. If the actual value is larger, the H-bridge is kept in its fast
demagnetization state, at the most until the end of the chopper period.
Hereby, the current is regulated to the desired value with a minimum of
current ripple. This reduces the electromagnetic radiation. Also, the
impact of current spikes, appearing when the H-bridge is switched, is
brought to a minimum.
Inventors:
|
Uggla; Dan J (Mossrosbacken, SE);
Wennergren; Rolf (.ANG.kervagen, SE)
|
Assignee:
|
New Japan Radio Co., Ltd. (Nihonbashi, JP)
|
Appl. No.:
|
376050 |
Filed:
|
August 17, 1999 |
Foreign Application Priority Data
Current U.S. Class: |
318/434; 318/254; 318/685; 318/696 |
Intern'l Class: |
H02K 017/32 |
Field of Search: |
318/365-367,375-376,379-380,434,759,760,685,696,254,70,280-300
|
References Cited
U.S. Patent Documents
4581565 | Apr., 1986 | Van Pelt et al. | 318/294.
|
5343382 | Aug., 1994 | Hale et al. | 363/98.
|
5372045 | Dec., 1994 | Schulz et al. | 73/861.
|
5457364 | Oct., 1995 | Bilotti et al. | 318/434.
|
5530639 | Jun., 1996 | Schulz et al. | 363/17.
|
5650705 | Jul., 1997 | Hart | 318/635.
|
5930103 | Jul., 1999 | Heck | 361/187.
|
5986418 | Nov., 1999 | Horst et al. | 318/254.
|
Primary Examiner: Sircus; Brian
Assistant Examiner: Duda; Rina I.
Attorney, Agent or Firm: Armstrong, Westerman, Hattori, McLeland & Naughton
Claims
What is claimed is:
1. A method of controlling a current through a winding interconnected in an
H-bridge by switching the H-bridge, during repetitive chopper periods,
between a magnetization state, a slow demagnetization state, and a fast
demagnetization state, characterized by the following steps during each
chopper period:
at the beginning of each chopper period, keeping the H-bridge in the slow
demagnetization state during a predetermined interval (t1-t2, t1'-t2');
comparing the actual value of the current through the winding with desired
value;
if the actual value is smaller than the desired value, keeping the H-bridge
in the magnetization state after said predetermined interval (t1-t2), at
the most until the end of the chopper period;
if the actual value is larger than the desired value, keeping the H-bridge
in the fast demagnetization state after said predetermined interval
(t1'-t2'), at the most until the end of the chopper period.
2. The method of claim 1, characterized by comparing the actual value with
the desired value just before, at or just after the end of said
predetermined interval (t1-t2, t1'-t2').
3. The method of claim 1, characterized by the step of keeping the H-bridge
in the magnetization state only until the actual value equals the desired
value during the chopper period after said predetermined interval (t1-t2),
if the actual value is smaller than the desired value when compared, and
thereafter keeping the H-bridge in the slow demagnetization state for the
rest of the chopper period.
4. The method of claim 1, characterized by the step of keeping the H-bridge
in the fast demagnetization state only for a predetermined period of time
(t2'-t3) after said predetermined interval (t1'-t2'), if the actual value
is larger than the desired value when compared, and thereafter keep the
H-bridge in the slow demagnetization state for the rest of the chopper
period.
5. An arrangement for controlling a current through a winding (L)
interconnected in an H-bridge switchable, during repetitive chopper
periods, between a magnetization state, a slow demagnetization state, and
a fast demagnetization state, characterized in
that a control circuit (1) is adapted, at the beginning of each chopper
period, to keep the H-bridge in the slow demagnetization state during a
predetermined interval (t1-t2, t1'-t2');
that a comparator (2) is adapted to compare the actual value (IA; IA') of
the current through the winding (L) with a desired value (ID; ID'), and to
inform the control circuit (1) accordingly;
that, if the actual value (IA) is smaller than the desired value (ID), the
control circuit (1) is adapted to keep the H-bridge in the magnetization
state after said predetermined interval (t1-t2), at the most until the end
of the chopper period, and
that, if the actual value (IA') is larger than the desired value (ID'), the
control circuit (1) is adapted to keep the H-bridge in the fast
demagnetization state after said predetermined interval (t1'-t2'), at the
most until the end of the chopper period.
6. The arrangement of claim 5, characterized in that the comparator (2) is
adapted to compare the actual value with the desired value just before, at
or just after the end of said predetermined interval (t1-t2; t1'-t2').
7. The arrangement of claim 5, characterized in that the control circuit
(1) is adapted to keep the H-bridge in the magnetization state only until
the actual value (IA) equals the desired value (ID) during the chopper
period after said predetermined interval (t1-t2), if the actual value (IA)
is smaller than the desired value (ID) when compared, and thereafter keep
the H-bridge in the slow demagnetization state for the rest of the chopper
period.
8. The arrangement of claim 5, characterized in that the control circuit
(1) is adapted to keep the H-bridge in the fast demagnetization state only
for a predetermined period of time (t2'-t3) after aid predetermined
interval (t1'-t2'), if the actual value (IA') is larger than the desired
value (ID') when compared, and thereafter keep the H-bridge in the slow
demagnetization state for the rest of the chopper period.
Description
BACKGROUND OF THE INVENTION
The invention relates generally to current control and more specifically to
a method and an arrangement for controlling a current through a winding
interconnected in an H-bridge for controlling e.g. a stepping motor.
As is well known, in order to control the current through the winding to a
desired value, such H-bridges are switched alternately during repetitive
chopper periods among a magnetization state in which the winding is
magnetized, a slow demagnetization state in which the winding is slowly
demagnetized, and a fast demagnetization state in which the winding is
quickly demagnetized.
If the desired value is constant or increasing, there are normally no
problems to control the current to the desired value.
However, if the desired value decreases substantially, it is more difficult
to control the current to the desired value.
A number of solutions to this problem are known.
The most common is to, in some way, sense whether the desired value
increases or decreases. If the desired value decreases more than some
specified criterion, the H-bridge is kept in the fast demagnetization
state. Otherwise, it is kept in the slow demagnetization state. A
disadvantage of this method is that a decision criterion for when fast
demagnetization is to be used, has to be defined for each application and
operational mode. This can be very time consuming. The method does not
either solve resonance problems that arise when the actual value of the
current oscillates.
According to another known method, an automatic choice is made between slow
and fast demagnetization. In this case, a clock pulse is used, which
always switches the H-bridge to the magnetization state. Just before,
during or just after the switching of the H-bridge to the magnetization
state, a comparison is made between the actual value and the desired value
of the current through the winding. The result of this comparison controls
in its turn whether fast or slow demagnetization is to be used after the
magnetization phase. The actual value of the current through the winding
increases during the magnetization phase. The actual value is compared to
a desired value and when the actual value reaches the desired value, the
H-bridge is switched to either the fast or the slow demagnetization state,
the choice being controlled by the previous current comparison. The choice
between the two demagnetization states can then be varied in accordance
with a couple of different methods up to the next clock cycle when
magnetization again is initiated.
The disadvantage of this method is that a relatively large current ripple
is obtained due to the fact that there is a short period during each clock
cycle when magnetization is carried out independent of both the actual
value and the desired value. By also controlling the time during which
fast demagnetization is used, the ripple can be improved to a certain
extent by that method. However, a requirement is that it should be
possible to measure the current through the winding irrespective of the
state of the H-bridge. In practice, this is quite difficult since the
current has to be measured in series with the winding. The potential in
the point where the current measurement has to take place, varies
strongly. Thus, this measurement method is rarely used.
As examples of the prior art, U.S. Pat. No. 4,908,562 and Swedish Patent
Application No. 9800131-6 can be mentioned.
SUMMARY OF THE INVENTION
The object of the invention is to eliminate the problems of the prior art
methods.
This is attained in accordance with the invention by, at the beginning of
each chopper period, keeping the H-bridge in the slow demagnetization
state during a predetermined interval. Then, just before, at or just after
the end of that interval, the actual value of the current through the
winding is compared with a desired value. If the actual value is smaller
than the desired value, the H-bridge is kept in the magnetization state
after said predetermined interval, at the most until the end of the
chopper period. On the other hand, if the actual value is larger than the
desired value, the H-bridge is kept in the fast demagnetization state
after said predetermined interval, at the most until the end of the
chopper period.
The current through the winding can easily be measured as a voltage over a
sense resistor which is connected between the H-bridge and ground in a
manner known per se.
By controlling the bridge in such a manner that it never tries to increase
the current when the actual value of the current is higher than the
desired value and vice versa, the current ripple is brought to a minimum.
Also, the electromagnetic noise produced by the winding is reduced to a
minimum.
When the H-bridge changes state, a current spike originating from electric
charges stored in recirculation diodes of the H-bridge, will inevitably
pass through the sense resistor. The switching sequence of the H-bridge is
controlled in such a manner that such spikes do not interfere with the
current regulation. This will increase the accuracy and speed of the
current regulation.
The switching of the H-bridge between slow and fast demagnetization states
is made automatically and is based on the difference between the actual
and the desired values of the current through the winding. Hereby,
resonance problems are reduced when the actual value of the current
oscillates.
The switching of the H-bridge is easily controlled. Thus, no time consuming
evaluation of the application is needed to optimize its performance.
BRIEF EXPLANATION OF THE DRAWINGS
The invention will be described more in detail below with reference to the
appended drawings, on which:
FIGS. 1(a) to 1(c) illustrate current flows through a winding in a H-bridge
in three different states of the H-bridge;
FIG. 2 is a diagram illustrating how the current through the winding is
controlled when the desired value of the current is constant; and
FIG. 3 is a diagram illustrating how the current through the winding is
controlled when the desired value of the current decreases substantially.
DETAILED DESCRIPTION
FIGS. 1(a) to 1(c) illustrate current flows through a winding L in a known
H-bridge in three different states of the H-bridge. Even if the H-bridge
can further be switched to other states, these three states only are used
in accordance with the invention.
In a manner known per se, the H-bridge shown in FIGS. 1(a) to 1(c)
comprises four transistors T1, T2, T3 and T4. The collector of the
transistor T1 is interconnected with the collector of the transistor T2
and the interconnection point is connected to a voltage terminal V. The
bases of the transistors T1 and T2 are each connected to an output
terminal of a control circuit 1.
The emitters of the transistors T1 and T2 are interconnected with the
collectors of the transistors T3 and T4, respectively, and the winding L
is interconnected between the interconnection points.
The emitters of the transistors T3 and T4 are interconnected, and the
interconnection point is connected to ground via a resistor R.
The bases of the transistors T3 and T4 are also connected to individual
output terminals of the control circuit 1.
A recirculation diode D1 is interconnected between the collector of the
transistor T1 and the interconnection point between the emitter of the
transistor T1 and the collector of the transistor T3.
In a similar manner, a recirculation diode D2 is interconnected between the
collector of the transistor T2 and the interconnection point between the
emitter of the transistor T2 and the collector of the transistor T4.
A recirculation diode D3 is interconnected between the interconnection
point between the emitter of the transistor T1 and the collector of the
transistor T3 and ground.
In a similar manner, a recirculation diode D4 is interconnected between the
interconnection point between the emitter of the transistor T2 and
collector of the transistor T4 and ground.
A comparator 2 is connected via a blanking circuit or a low pass filter
(not shown) to the interconnection point between the emitters of the
transistors T3 and T4 to sense the voltage across the resistor R, and
convert that voltage into a value of the current through the resistor R,
i.e. the actual value of the current through the winding L, and compare it
with a desired value supplied to the comparator 2 via an input terminal 3
of the comparator 2 in a manner known per se from e.g. a motor control
unit (not shown). The comparator 2 informs the control circuit 1 about the
result of the comparison via an output terminal connected to an input
terminal of the control circuit 1.
The purpose of the blanking circuit or low pass filter (not shown) is to
eliminate the current spike that always occurs across resistor R when the
H-bridge changes state. The origin of the current spike is mainly electric
charges stored in the recirculation diodes D1, D2, D3 and D4. A minor
current spike occurs from electric charges stored in the capacitance
between collector and emitter of the transistors T1, T2, T3 and T4 and the
capacitance of the winding.
The switching sequence in accordance with the invention is started in such
a manner that no current spike, originating from the recirculation diodes,
is produced across the resistor R when the H-bridge enters its
magnetization state and starts to regulate the actual current through the
winding to the desired value. This makes it possible to use a much shorter
blanking time of the blanking circuit (not shown) or a low pass filter
(not shown) having a larger bandwith. This will in turn increase the
accuracy and speed in the current regulation.
FIG. 1(a) illustrates the case when the H-bridge is in the state when the
winding L is magnetized. In this magnetization state of the H-bridge, the
transistor T1 and T4 are on under control by the control circuit 1. Thus,
current will flow through the transistor T1, the winding L, the transistor
T4 and the resistor R to ground as schematically indicated by a solid line
4 in FIG. 1(a). The magnetization current, i.e. the actual current through
the winding L, is sensed as a voltage across the resistor R.
In FIGS. 1(b) and 1(c), the control circuitry for the H-bridge is not
shown.
FIG. 1(b) illustrates the case when the H-bridge is in the state when the
winding L is slowly demagnetized. In this slow demagnetization state, the
transistor T4 is on under control by the control circuit 1 as illustrated
in FIG. 1(a). Thus, the demagnetization current will flow from the winding
L, through the transistor T4, the resistor R, and the recirculation diode
D3 back to the winding L as illustrated by means of a solid line 5 in FIG.
1(b). As in FIG. 1(a) the actual current through the winding L, i.e. in
this case the demagnetization current, is sensed by the comparator 2 in
FIG. 1(a) as a voltage across the resistor R.
In this slow demagnetization state, one transistor should be on. One of the
transistors closest to the resistor R should be chosen. In FIG. 1(b) the
transistor T4 was chosen to be on by the control circuit 1 in FIG. 1(a).
However, the transistor T3 could equally well have been chosen to be on.
In that case, the current would flow in the opposite direction, i.e. from
the winding L, through the transistor T3, the resistor R and the
recirculation diode D4 back to the winding L.
FIG. 1(c) illustrates the case when the H-bridge is in the state when the
winding L is quickly demagnetized. In this fast demagnetization state,
none of the transistors T1-T4 is on. In this state, current will flow
through the recirculation diode D3, the winding L and the recirculation
diode D2 as illustrated by means of a solid line 6 in FIG. 1(c). The
actual current, i.e. the current flow as represented by the solid line 6,
can not be sensed across the resistor R in this case.
In accordance with the invention, the H-bridge is switched, during
repetitive chopper periods, between the different states, as illustrated
in FIGS. 1(a) to 1(c) to control the value of the actual current through
the winding L in respect of a desired current value that may vary, and
that is supplied to the input terminal 3 of the comparator 2 from e.g. a
motor control unit as mentioned above.
FIG. 2 illustrates the case when the desired value ID of the current
through the winding L, is supposed to be constant. It should be pointed
out that FIG. 2 is valid also when the desired value ID is increasing.
In accordance with the invention, at the beginning of each chopper period,
i.e. at a time t1, the H-bridge, by means of the control circuit 1, is
always controlled to its slow demagnetization state as represented by FIG.
1(b), and is kept in this state for a predetermined period of time ending
at a time t2as shown in FIG. 2. In this state of the H-bridge, the actual
value IA of the current through the winding L will decrease slowly as
apparent from FIG. 2.
In view of the fact that quite a large current spike originating from
charges stored in the recirculation diodes, will occur across the resistor
R when the H-bridge goes from its fast demagnetization state, as
illustrated in FIG. 1(c), to its slow demagnetization state, as
illustrated in FIG. 1(b), the time t1-t2 should be longer than the length
of this current spike. Thereby, that inevitable current spike will not
interfere with the current regulation.
Just before, at or just after the end of the predetermined interval t1-t2,
i.e. around the time t2, the actual value IA of the current through the
winding L sensed across the resistor R by the comparator 2, is compared
with the desired value ID supplied to the comparator via the input
terminal 3.
In FIG. 2, the actual value IA of the current through the winding L is
supposed to be smaller than the desired value ID.
This is detected by the comparator 2 which informs the control circuit 1
accordingly.
In this case, the control circuit 1 brings the H-bridge to the
magnetization state as represented by FIG. 1(a), and keeps it in that
state after the predetermined interval, i.e. after the time t2, at the
most until the end of the chopper period in question, i.e. until the next
time t1.
However, if the actual value IA reaches the desired value ID during the
rest of the chopper period in question, as in FIG. 2, this is detected by
the comparator 2. The comparator 2 informs the control circuit 1
accordingly, and the control circuit 1 brings the H-bridge again to its
slow demagnetization state for the rest of the chopper period, i.e. to the
next time t1.
When the next chopper period starts at time t1, since the H-bridge already
is in its slow demagnetization state in FIG. 2, it will stay in that state
until after the next time t2.
Should the H-bridge for some reason be in its magnetization state when the
next chopper period starts at the next time t1, in accordance with the
invention as above, the H-bridge will be brought by the control circuit 1
to the slow demagnetization state at the beginning of that next chopper
period, i.e. at said next time t1.
FIG. 3 illustrates the case when the desired value of the current through
the winding L decreases substantially, and the actual value is larger than
the desired value. In FIG. 3, the desired value is denoted ID' and the
actual value is denoted IA'.
As in FIG. 2, and in accordance with the invention, at the beginning of
each chopper period, in FIG. 3 at times t1', the control circuit 1 brings
the H-bridge to the slow demagnetization state as represented by FIG.
1(b), and keeps it in that state during a predetermined period of time, in
FIG. 3 until time t2'.
Just before, at or just after the end of that predetermined period of time,
i.e. around time t2', the comparator 2 compares the actual value IA' of
the current through the winding L, as calculated from the voltage sensed
across the resistor R, with the desired value ID', as supplied via the
input terminal 3 from e.g. a motor control unit, and detects that the
actual value IA' is larger than the desired value ID'.
Then, the comparator 2 informs the control circuit 1 accordingly and the
control circuit 1 brings the H-bridge to the fast demagnetization state as
represented by FIG. 1(c) at time t2', and keeps the H-bridge in that fast
demagnetization state, at the most until the end of the chopper period in
question, i.e. to the next time t1'.
Normally, as illustrated in FIG. 3, the H-bridge will be kept in its fast
demagnetization state until the beginning of the next chopper period, i.e.
the next time t1', when the H-bridge again is brought to the slow
demagnetization state until the next time t2'.
However, in order to avoid too quick a demagnetization of the winding L and
thereby also restrict the current ripple, the H-bridge may be kept in its
fast demagnetization state only for a second predetermined period of time,
e.g. from time t2' until a time t3 as illustrated in FIG. 3. From that
time t3 until the end of the chopper period at time t1', the H-bridge is
kept in its slow demagnetization state, as indicated in FIG. 3 by a broken
line IA".
The time t3 could be either fixed or controlled e.g. from the difference
between the actual and desired values of the current. Namely, as is
illustrated in relation to FIG. 2, t3 could be so controlled as to be a
time when the actual value of the current reaches the desired value. Also
in this case, the longest time of t3 is limited within the rest of the
chopper period, i.e. by the next time t1', in accordance with the control
rule of the invention.
To sum up, by always starting the chopper period with the H-bridge in its
slow demagnetization state for a predetermined time, has two main
advantages.
First, the large current spike originating from the recirculation diodes,
will not interfere with the current regulation. Thereby, the current can
be regulated to the desired value with higher speed and accuracy.
Second, the H-bridge is switched between its slow and its fast
demagnetization states only if the desired value of the current through
the winding is lower than the actual value, and is switched between its
slow demagnetization state and its magnetization state only when the
desired value of the current through the winding is higher than the actual
value. Hereby, the current ripple and the electromagnetic noise will be
reduced to a minimum.
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